Welcome to askubuntu! Try sudo apt-get install unity-webapps-gmail in order to see if this isn't a bug. After this open the Dash and search for gmail. Also edit your question to include the output of gsettings list-recursively | grep unity\.webapps. (You might also try seeing that Firefox is your default web browser and that in its Preferences, General tab, the option Prompt integration options for any website is checked.)
–
edwinJul 5 '13 at 19:39